Bromination of alkylbenzenes in 1-butyl-3-methylimidazolium bromide and its dibromide complex

Article abstract of DOI:10.1134/S1070363210020143

Alkylbenzenes were subjected to bromination with molecular bromine using 1-butyl-3-methylimidazolium bromide as solvent. A complex of 1-butyl-3-methylimidazolium bromide with bromine was synthesized. It ensured bromination of alkylbenzenes with no bromine and solvent. The results of bromination in binary solvents and ionic liquids, 1-butyl-3-methylimidazolium bromide and tribromide were compared. The bromination of ethylbenzene with 1-butyl-3-methylimidazolium tribromide was accompanied by formation of a considerable amount of α-bromoethylbenzene, which is not typical of electrophilic aromatic substitution process.

Full text of DOI:10.1134/S1070363210020143

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 2, pp. 263–267. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © M.S. Gruzdev, L.V. Virzum, E.N. Krylov, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 2, pp. 238–242.
Bromination of Alkylbenzenes in 1-Butyl-3-methylimidazolium
Bromide and Its Dibromide Complex
a
b
c
M. S. Gruzdev , L. V. Virzum , and E. N. Krylov
a
Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: gms@isc-ras.ru
b
Belyaev Ivanovo State Agricultural Academy, Ivanovo, Russia
c
Ivanovo State University, Ivanovo, Russia
Received April 2, 2009
Abstract—Alkylbenzenes were subjected to bromination with molecular bromine using 1-butyl-3-
methylimidazolium bromide as solvent. A complex of 1-butyl-3-methylimidazolium bromide with bromine was
synthesized. It ensured bromination of alkylbenzenes with no bromine and solvent. The results of bromination
in binary solvents and ionic liquids, 1-butyl-3-methylimidazolium bromide and tribromide were compared. The
bromination of ethylbenzene with 1-butyl-3-methylimidazolium tribromide was accompanied by formation of a
considerable amount of α-bromoethylbenzene, which is not typical of electrophilic aromatic substitution
process.
DOI: 10.1134/S1070363210020143
A foreground field of modern “green chemistry” is
the use of ionic liquids [1], i.e., salts formed by
heterocyclic bases (cations) with acids (anions), as
promising media for carrying out organic reactions. It
is expected that the use of ionic liquids will improve
the rate, conversion, yield, and selectivity of organic
reactions, especially of catalytic processes [2–6]. Ionic
liquids are believed to be highly effective and selective
reagents in electrophilic substitution reactions [7]. In
addition, the use of ionic liquids provides solution of
an important problem of organic chemistry, namely
regeneration of solvent and catalyst. Ionic liquids may
be repeatedly used in organic syntheses, which makes
it possible to reduce toxicity of a number of organic
compounds [8].
[9], and a combination of aqueous tert-butyl hydro-
peroxide or hydrogen peroxide with hydrohalic acid
[10] were used.
It was shown [11–14] that in some cases the use of
ionic liquids as reagent and solvent ensures stereo-
selective synthesis of aromatic alcohols and ethers, as
well as of compounds belonging to other classes. Ion
liquid-based procedures make it possible to avoid
formation of by-products, improve regio- and stereo-
selectivity, and simultaneously accelerate electrophilic
substitution processes.
With a view to examine the selectivity of elec-
trophilic substitution in ionic liquids, as model reaction
we selected electrophilic bromination of alkylben-
zenes, taking into account that the latter are fairly
reactive toward electrophiles and that the bromination
products can readily be analyzed for isomeric com-
position. Bromination of alkylbenzenes gives rise to
mixtures of isomeric ortho-, meta-, and para-bromo-
substituted alkylbenzenes at different ratios which
depend on the nature and size of alkyl group, solvent,
and reagent [15].
Among electrophilic substitution reactions, bro-
mination, nitration, and sulfonation were studied most
thoroughly. Halogen derivatives of the aromatic series
are key intermediate products in organic synthesis;
they are precursors of numerous organometallic
compounds used in the synthesis of natural products
and pharmaceutical agents. Direct bromination of
aromatic systems is usually performed with molecular
bromine which generares toxic and corrosive hydrogen
bromide. Also, catalysts based on expensive transition
The bromination of alkylbenzenes was carried out
in the dark using 1-butyl-3-methylimidazolium
bromide ([BMIm]Br) as solvent and catalyst and
metals, alkali metal halides in the presence of NaIO
4
2
63

2
64
GRUZDEV et al.
R
R
R
R
Br
3
+ 3 Br2
+
+
+ 3 HBr
Br
Br
molecular bromine as brominating agent. Unlike
bromination with molecular bromine, the reaction with
BMImBr gave a mixture of o- and p-bromoethyl-
benzenes (Table 1).
the bromination of ethylbenzene in different solvents
was consistent with usual positional selectivity of this
reaction [16].
In going from nitromethane to acetic acid
containing some water, the degree of ortho substitution
increases from 24 to 32–34%, whereas the yield of o-
bromoethylbenzene in ionic liquid attains 38%.
Simultaneously, the fraction of the meta-substituted
isomer falls down to almost zero, which is not typical
of electrophilic substitution in alkylbenzenes. The
isomer composition of the products suggests that the
bromination of ethylbenzene in organic solvents and
R
R
R
Br
BMImBr
Br2
2
+
+ 2 HBr
2
Br
The results of bromination of ethylbenzene using 1-
ionic liquid follows the classical S 2Ar mechanism.
E
butyl-3-methylimidazolium tribromide ([BMIm]Br )
3
as solvent and reagent are also given in Table 1. For
comparison, the data on bromination of the same
substrate with bromine in aqueous acetic acid and
nitromethane are presented. The ratio of the major
substitution products, o- and p-bromoethylbenzenes, in
In the bromination of ethylbenzene with 1-butyl-3-
methylimidazolium tribromide, the major product was
the corresponding α-bromo derivative, indicating
predominant contribution of the radical mechanism
(Table 1).
Me
N
−
+
Br
3
CH CH
2
CH CH3
2
3
CH CH
2
3
CH CH Br
2 2
N
Br
Bu
3
+
+
Br
The presence of a bromine molecule in the complex
[BMIm]Br (Fig. 1). Presumably, bromine in [BMIm]Br
3
[
BMIm]Br weakly affects the position and intensity of
occurs in a bound state, but it nevertheless is suf-
ficiently labile to halogenate alkylbenzenes.
3
absorption bands in the IR spectra, as compared to
Br
CH3
CH3
R
R
N
N
+
HBr−
+
−
Br3
2
N
N
CH(CH ) CH
3
CH(CH ) CH
2 2
2
2
3
HBr
Br2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 2 2010

BROMINATION OF ALKYLBENZENES IN 1-BUTYL-3-METHYLIMIDAZOLIUM BROMIDE
265
Table 1. Isomer composition (%) of bromoethylbenzenes in
the bromination of ethylbenzene in organic solvents and
ionic liquid; temperature 50°C
Table 2. Isomer composition (%) of bromoethylbenzenes in
the bromination of ethylbenzene in organic solvents and 1-
butyl-3-methylimidazolium tribromide [BMIm·Br
perature 50°C, reaction time 2.5 h
3
]; tem-
Solvent
NO
ortho
24.3
32.5
34.2
34.0
38.2
4.4
meta
2.5
2.4
2.8
2.3
–
para
73.2
65.4
0.57
63.7
61.8
7.5
α
–
log (2p/o)
0.78
Methyl-
benzene
Ethyl-
benzene
Isopropyl-
benzene
Solvent
Isomer
CH
3
2
ortho
meta
para
43.4
1.7
54.9
32.5
2.4
65.4
22.0
3.9
74.1
AcOH (70%)
AcOH (85%)
AcOH (99.5%)
–
0.61
AcOH (70%)
–
63.0
ortho
meta
para
ortho
meta
para
ortho
meta
para
40.0
2.0
34.2
2.8
20.1
4.5
–
0.57
AcOH (85%)
AcOH (99.5%)
Nitromethane
58.0
63.0
75.4
[
BMIm·Br]
BMIm·Br
–
0.51
44.0
1.4
55.6
35.6
1.8
62.6
34.0
2.3
63.7
24.3
2.5
73.2
18.4
4.0
77.6
15.2
4.2
80.6
[
3
]
0.8
87.3
0.53
Comparison of the data on positional selectivity in
the bromination of alkylbenzenes in organic solvents
and [BMIm]Br (Table 2, Fig. 2) indicates determining
3
ortho
meta
para
α
47.2
0
50.2
2.6
4.4
0.8
8.7
9.4
16.3
65.6
role of steric effects of the substituents. The existence
of linear correlation suggests inversely proportional
dependence of the ratio of para and ortho isomers
upon alkyl substituent steric constants according to
Pal’m–Koppel’ [17]. The sensitivity of ortho/para
orientation to the effective size of alkyl substituent in
3
[BMIm·Br ]
7.5
87.3
mechanism of the process, though all reactions were
carried out with protection from light [18]. Classical
electrophilic substitution mechanism is usually
assumed in the halogenation of aromatic compounds in
1
-butyl-3-methylimidazolium tribromide is considerably
weaker and is nonlinear. High polarity of the medium
increase the role of electronic effect of substituents
whose donor power increases in going from methyl to
isopropyl group.
ionic liquids. The complex [BMIm]Br is believed to
3
be a highly selective reagent in the bromination of
organic compounds; in particular, the bromination of
phenol gives 90% of p-bromophenol [19]. This
problem requires special study, for there is no relation
between the degree of α-substitution in alkylbenzenes
As shown above, the bromination of ethylbenzene
with [BMIm]Br is characterized by anomalously high
3
degree of α-substitution, which is typical of radical
1
1
0
0
0
0
0
0
0
.1
.0
.9
.8
.7
.6
.5
.4
.3
4
3
2
1
2
1
5
–
1.0
–0.8
–0.6
–0.4
–0.2
0.0
0
E
S
3
750
3000
2250
ν, cm
1500
750
Fig. 2. Plot of log (2p/o) (parameter characterizing positional
selectivity) versus the size of alkyl substituent in the
bromination of alkylbenzenes in (1) 70% acetic acid (2) 85%
acetic acid, (3) 99.5% acetic acid, (4) nitromethane, and
(5) 1-butyl-3-methylimidazolium tribromide.
–
1
Fig. 1. IR spectra of (1) 1-butyl-3-methylimidazolium
bromide and (2) 1-butyl-3-methylimidazolium tribromide.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 2 2010

2
66
GRUZDEV et al.
and the nature of the alkyl group (Me, Et, i-Pr)
49.62, 122.53, 123.89, 136.67. Found, %: C 42.16;
(
Table 2).
H 7.50; N 13.38; Br 37.96. C Br. Calculated, %:
8
H
15
N
2
C 43.58; H 7.47; N 12.70; Br 36.24.
The use of 1-butyl-3-methylimidazolium tribromide
makes electrophilic bromination of alkylbenzenes
1-Butyl-3-methylimidazolium tribromide was
experimentally simpler and safe, for [BMIm]Br acts
synthesized by reaction of equimolar amounts of
3
simultaneously as solvent and reagent. This is also
advantageous from the ecological viewpoint. Rege-
BMImBr and Br [21]. Yield 85 g (97.3%), bright red
2
–
1
liquid. IR spectrum, ν, cm : 3147–3081 s (C–H ),
arom
neration and repeated use of [BMIm]Br without
2958–2876 (C–H ), 1573 (C–N), 1169, 622 (amide
3
aliph
1
additional purification are also possible.
III). H NMR spectrum, δ, ppm: 0.98 t (3H, CH
3
, J =
7
.34 Hz), 1.42 sext (2H, CH CH , J = 14.65 Hz), 1.95
3
2
EXPERIMENTAL
q (2H, NCH CH , J = 15.26 Hz), 3.93 s (3H, NCH ),
2
2
3
4
.35 t (2H, NCH , J = 7.32 Hz), 7.56 s (1H, 5-H), 7.6 s
2
1
3
All solvents and reagents were of chemically pure
or ultrapure grade and were used without additional
purification. Isomer composition of bromoalkyl-
benzenes was determined by GLC on an LKhM-80
chromatograph equipped with a model 6 flame
ionization detector; carrier gas hydrogen, flow rate
(1H, 4-H), 9.48 s (1H, 2-H). C NMR spectrum, δ ,
C
ppm: 13.55, 19.57, 32.15, 37.08, 50.12, 122.47,
123.88, 136.49. The product was stored in the dark and
used without additional purification.
Typical procedure for bromination of alkyl-
benzenes. A required amount of alkylbenzene was
placed into a dark glass bottle with a ground stopper,
which was charged with ionic liquid. Bromobenzene
–
1
3
0 ml min ; 3000 ×3-mm column packed with XE-60
polycyanosiloxane on Chezasorb N-AW-HMDS; oven
temperature 90–130°C, detector temperature 120°C,
injector temperature 120°C. The IR spectra (350–
(
internal standard) was then added, and the mixture
–
1
was kept at 50°C. Molecular bromine was added from
a vessel maintained at 50°C, and the mixture
was continuously stirred. The molar ratio
4
500 cm ) were recorded on an Avatar 360 FT-IR ESP
spectrometer; samples were prepared as KBr pellets
1
13
and placed between KRS plates. The H and C NMR
spectra were measured on a Bruker AC-200 instrument
at 200.13 and 50.32 MHz, respectively, using chloro-
form-d as solvent and tetramethylsilane as internal
reference. The elemental compositions of crystalline
compounds were determined on a FlashEA 1112
analyzer. The mass spectra were obtained on a Saturn
EtPh:PhBr:BMImBr:Br was 1:0.01:1:0.5. In the
2
bromination with [BMIm]Br , the latte was added to a
3
mixture of alkylbenzene and internal standard, and the
mixture was stirred at 50°C. The molar ratio
EtPh:BrPh:[BMIm]Br was 1:0.01:0.5, and the reac-
3
tion time was 2.5 h.
2
000R GC–MS system using helium as carrier gas.
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-Butyl-3-methylimidazolium
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